The antiviral activity of thiosemicarbazones was first reported in 1950 by Hamre et al, in Proc. Soc. Exp. Biol. Med., Vol. 73, pp. 275-278. They found that derivatives of benzaldehyde thiosemicarbazone were active against neurovaccinial infection in mice when given orally. This prompted further investigation of other thiosemicarbazones. The thiosemicarbazone of isatin was found by Bauer in 1955 to be one of the most active as reported in British J. Exp. Pathology, Vol. 36, pp. 105-114, and a clinical trial of the N-methyl derivative of isatin-beta-thiosemicarbazone (methisazone) was carried out by Bauer and Associates in India. The studies indicated that the drug was effective in the prevention of smallpox in persons exposed to the disease. Although these studies have been widely accepted as evidence of the effective antiviral activity of methisazone in humans, a more recent field trial study reported on in 1971 by Heiner et al, Am. J. Epidemiology, Vol. 94, pp. 435-449, demonstrated little efficacy in humans. The drug also has been used to treat patients with genital lesions caused by herpes simplex virus (HSV), but it had little effect on the severity or duration of the lesions as reported in 1964 by Hutfield et al in Lancet, Vol. 1, pp. 329-330.
In 1969, Sidwell and co-workers evaluated a series of purine analogs as antiviral agents and reported in Proc. Soc. Exp. Biol. Med., Vol. 131, pp. 1223-1230 that purine-6-carboxaldehyde thiosemicarbazone was effective in suppressing both the cytopathic effect and the titers of human cytomegalovirus. This was the first report of a substituted thiosemicarbazone being active against a herpesvirus. In 1970, the effect of heterocyclic thiosemicarbazones was examined by Brockman et al and reported in Proc. Soc. Exp. Biol. Med., Vol. 133, pp. 609-614. They tested the effect of several pyridine, isoquinoline, purine, and isatin derivatives on HSV and found that only those compounds in which the thiosemicarbazide moiety was affixed to the heterocyclic ring in the alpha position to the ring nitrogen were active. For example, beta-isatin, 3-formylpyridine, and 4-formylpyridine derivatives were inactive, whereas the 2-formylpyridine derivative was active.
According to the instant invention, applicants have demonstrated that selected 2-acetylpyridine thiosemicarbazones inhibit the replication of herpes simplex virus types 1 and 2 to a greater extent than cellular DNA (deoxyribonucleic acid) or protein synthesis. By using CsCl density gradient ultracentrifugation, we have been able to show that for the derivatives tested, viral DNA synthesis was inhibited to a greater extent than cellular DNA synthesis.